1. Technical Field
This relates to a clock circuit for microprocessors. It is particularly well-suited for use in Personal Digital Assistants, mobile communication devices, cellular phones, and wireless two-way email communication devices (collectively referred to herein as “mobile devices”). It may provide utility, however, in any device that is subjected to high levels of electromagnetic interference.
2. Description of the Related Art
Known clock circuits commonly include crystal oscillators that resonate at a certain frequency. Once the crystal oscillator begins vibrating at its resonant frequency, the resonant frequency is typically maintained by feeding back an in-phase signal from one terminal of the crystal oscillator to the other terminal of the crystal oscillator. This allows the clock circuit to generate a substantially constant clock speed.
Known clock circuits generally have high impedances in order to reduce power consumption. The clock circuit's high impedance, however, makes it susceptible to interference from strong electromagnetic signal sources. For example, in a mobile device, a transmission circuit may include a power amplifier that interferes with the clock circuit during transmission, temporarily changing the frequency of the oscillator.
Typical mobile devices isolate the clock circuit from the strong electromagnetic interference that occurs during transmission by shielding the clock circuit with an EMI shield. In addition, a typical mobile device may include an external buffer amplifier that protects the integrity of the clock circuit. These known techniques generally add complexity and high component counts to the device. Furthermore, these techniques may require the circuit to restart the crystal oscillator in the event that the oscillation has stopped or slowed as a result of a strong interference signal.